Phototransduction (intracellular voltage response and voltage clamped photocurrent response) will be studied in Limulus ventral photoreceptors. The time course of the response is determined by kinetics of underlying chemical and physical events: Linear (sinusoidal) input-output analysis and nonlinear systems identification will be used to discribe photoresponse kinetics as they are affected by 1) steady state adapting light intensity; 2) change in adapting light intensity (thus the time course of light adaptation and dark adaptation as they affect kinetics will be measured); 3) alteration of ionic milieu both internally (via intracellular injection of chemicals or ions) and externally. A nonlinear model will be constructed from the transfer functions determined at each level of light adaptation. Amplitude nonlinearities in the photovoltage kinetics will be characterized by comparison to voltage clamp current records. Time dependent nonlinearities will be characterized by looking at the time course of change in transfer function observed when the adapting light level is changed. Results of linear systems analysis extended in this manner to the nonlinear domain will be compared with the results of nonlinear systems analysis including white noise and poisson flash train methods. Change in linear and nonlinear kinetics caused by alteration of external and internal calcium concentration, by change in temperature, and by modification of cyclic nucleotide metabolism will be measured. Resulting data will bear quantitative comparison to kinetics of chemical models which purport to describe photoresponse generation.